In this class belong—

Steam-engines.
Caloric or air engines.
Water-wheels or water-engines.
Wind-wheels or pneumatic engines.

These four types comprehend the motive-power in general use at the present day. In considering different engines for motive-power in a way to best comprehend their nature, the first view to be taken is that they are all directed to the same end, and all deal with the same power; and in this way avoid, if possible, the impression of there being different kinds of power, as the terms water-power, steam-power, and so on, seem to imply. We speak of steam-power, water-power, or wind-power; but power is the same from whatever source derived, and these distinctions merely indicate different natural sources from which power is derived, or the different means employed to utilise and apply it.

Primarily, power is a product of heat; and wherever force and motion exist, they can be traced to heat as the generating element: whether the medium through which the power is [30] obtained be by the expansion of water or gases, the gravity of water, or the force of wind, heat will always be found as the prime source. So also will the phenomenon of expansion be found a constant principle of developing power, as will again be pointed out. As steam-engines constitute a large share of the machinery commonly met with, and as a class of machinery naturally engrosses attention in proportion, the study of mechanics generally begins with steam-engines, or steam machinery, as it may be called.

The subject of steam-power, aside from its mechanical consideration, is one that may afford many useful lessons, by tracing its history and influence, not only upon mechanical industry, but upon human interests generally. This subject is often treated of, and both its interest and importance conceded; but no one has, so far as I know, from statistical and other sources, ventured to estimate in a methodical way the changes that can be traced directly and indirectly to steam-power.

The steam-engine is the most important, and in England and America best known among motive agents. The importance of steam contrasted with other sources of motive-power is due not so much to a diminished cost of power obtained in this way, but for the reason that the amount of power produced can be determined at will, and in most cases without reference to local conditions; the machinery can with fuel and water be transported from place to place, as in the case of locomotives which not only supply power for their own transit, but move besides vast loads of merchandise, or travel.

For manufacturing processes, one importance of steam-power rests in the fact that such power can be taken to the material; and beside other advantages gained thereby, is the difference in the expense of transporting manufactured products and the raw material. In the case of iron manufacture, for example, it would cost ten times as much to transport the ore and the fuel used in smelting as it does to transport the manufactured iron; steam-power saves this difference, and without such power our present iron traffic would be impossible. In a great many manufacturing processes steam is required for heating, bleaching, boiling, and so on; besides, steam is now to a large extent employed for warming buildings, so that even when water or other power is employed, in most cases steam-generating apparatus has to be set up in addition. In many cases waste [31] steam or waste heat from a steam-engine can be employed for the purposes named, saving most of the expense that must be incurred if special apparatus is employed.

Other reasons for the extended and general use of steam as a power, besides those already named, are to be found in the fact that no other available element or substance can be expanded to a given degree at so small a cost as water; and that its temperature will not rise to a point injurious to machinery, and, further, in the very important property of lubrication which steam possesses, protecting the frictional surfaces of pistons and valves, which it is impossible to keep oiled because of their inaccessibility or temperature.

The steam-engine, in the sense in which the term is employed, means not only steam-using machinery, but steam-generating machinery or plant; it includes the engine proper, with the boiler, mechanism for feeding water to the boiler, machinery for governing speed, indicators, and other details.

An apprentice must guard against the too common impression that the engine, cylinder, piston, valves, and so on, are the main parts of steam machinery, and that the boiler and furnace are only auxiliaries. The boiler is, in fact, the base of the whole, that part where the power is generated, the engine being merely an agent for transmitting power from the boiler to work that is performed. This proposition would, of course, be reached by any one in reasoning about the matter and following it to a conclusion, but the fact should be fixed in the mind at the beginning.

When we look at a steam-engine there are certain impressions conveyed to the mind, and by these impressions we are governed in a train of reflection that follows. We may conceive of a cylinder and its details as a complete machine with independent functions, or we can conceive of it as a mechanical device for transmitting the force generated by a boiler, and this conception might be independent of, or even contrary to, specific knowledge that we at the same time possessed; hence the importance of starting with a correct idea of the boiler being, as we may say, the base of steam machinery.

As reading books of fiction sometimes expands the mind and enables it to grasp great practical truths, so may a study of abstract principles often enable us to comprehend the simplest forms of mechanism. Even Humboldt and Agassiz, it is said, [32] resorted sometimes to imaginative speculations as a means of enabling them to grasp new truths.

In no other branch of machinery has so much research and experiment been made during eighty years past as in steam machinery, and, strange to say, the greater part of this research has been directed to the details of engines; yet there has been no improvement made during the time which has effected any co............